Differential excitation spectroscopy

ABSTRACT

A new technique which uses a pump-probe methodology to place a molecule into one or more excited rotational and/or vibrational states. By evaluating spectral changes due to at least one discrete frequency of pump photons a multi-dimensional characterization of the molecule&#39;s excited state energy level results. This multi-dimensional characterization typically involves evaluating the changes between excited and unexcited state measurements. The differential nature of the evaluation makes the technique self-referencing and solves problems common to many spectroscopic techniques. The multi-dimensionality of the technique provides high specificity and immunity to interferents. The preferred embodiments involve excitation by using photons suited to pumping the rotational states and evaluating the effects by probing the energy levels of one of more vibrational states. The technique is capable of detecting bulk and trace concentrations of a molecule in gas, liquid and solid phases, both in pure form and in the presence of other molecules.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of provisional applicationSer. No. 61/877,144, entitled “Differential Microwave ExcitationInfrared Spectroscopy”, filed Sep. 12, 2013. This application claims thepriority to and the benefit of such application. The disclosure thereofis incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a new spectroscopic technique calledDifferential Excitation Spectroscopy (DES), which uses a pump-probemethodology to place a molecule into one or more excited rotationaland/or vibrational states (hereafter collectively referred to as“rovibrational” states). By evaluating the spectral changes due to theone or more discrete frequencies of pump photons, instead of the onedimensional measure of a molecule (a spectral response curve) that iscommon to many spectroscopic techniques, a multi-dimensionalcharacterization of the molecule's excited state energy level structureresults. This multi-dimensional characterization typically involvesevaluation the changes between excited state (or perturbed) andunexcited (or base) state measurements; the differential nature of theevaluation makes the technique self-referencing and solves many problemscommon to many spectroscopic techniques. The multi-dimensionality of thetechnique provides high specificity and immunity to interferents. Thepreferred embodiments involve excitation by using photons suited topumping the rotational states and evaluating the effects by probing theenergy levels of one or more vibrational states. The technique iscapable of detecting both bulk and trace concentrations of a molecule inthe gas, liquid and solid phases, both in pure form and in the presenceof other molecules.

BACKGROUND

Presently, numerous options exist for chemicals and materials detectioninvolving laboratory and field-based monitoring, verification andaccounting (MVA) sensors and techniques that can quantify emissions. MVAtechniques include atmospheric monitoring technologies, remote sensingand near-surface monitoring technologies, and intelligent monitoringconcepts. Specific technological approaches include: atmospheric pointsamplers (“sniffers” based upon a wide-range of techniques such aselectrochemical (membrane), infrared, semiconductor, and ionization/ionmobility); eddy covariance (a.k.a., eddy correlation and eddy flux,including fiber optic sensor arrays based upon photonic bandgap (PBG));gas chromatography (GC) and accelerator mass spectroscopy (AMS)techniques (including thermal desorption, chemoluminescence,time-of-flight mass spectrometry techniques); acoustic wave andultrasonic detection; photoacoustic spectroscopy; laser fluorosensor(LFS) (fluorescence energy measurement); various Raman scatteringtechniques; gamma-ray spectroscopy; laser holographic sensing; varioussatellite and airborne sensors; and spectroscopic techniques such asback-scatter Light Detection and Ranging (LIDAR), laser-basedDifferential Absorption LIDAR (DIAL), and Differential OpticalAbsorption Spectroscopy (DOAS).

However, despite the large number of possible detection technologies, anumber of challenges remain to be addressed: (1) the environmentalbackground flux which continues to adversely affect detectionsensitivity; (2) turning measurements into an appropriatearea-integrated, mass balance (quantity) is difficult; (3) small “patch”area samples which are not ideally suited for cost-effectively andcomprehensively observing a large area; and (4) the statistical “spatialresolution” of present monitoring systems is too coarse and, thus theyare unable to easily (e.g., rapidly) locate and characterize anindividual hazard (e.g., threat) within the larger landscape (e.g.,separate one contaminated vehicle out of many uncontaminated vehicles).

With specific regard to detection sensitivity and operating in the realworld, many anthropogenic emissions are present that negatively affectrelevant measurements systems. For example: normal vehicle emissionssuch as ammonia (NH₃); carbon black from tires and combusted diesel;production of electricity; cement, chemical/fertilizer, mining andethanol emissions; pollen and attractants from certain flowering plants;volatile organic compounds (VOCs); farming and ranching practices suchas pesticide and herbicide application; and fine particulate matter inthe air. Furthermore, natural skin oils (e.g., squalene), chemicals usedin processed food (e.g., binders and preservatives), certainsoaps/shampoos, deodorants/antiperspirants, perfumes/colognes, andinsect repellents are all known to confuse or unfavorably affect thesensitivity of many detection techniques. In these environments andsituations not only is the detection of a specific gaseous,vapor/aerosol, solid or liquid species complicated by the background andcontaminates present, but these naturally occurring and anthropogenicsources of interferents will spoof many present detection techniqueswith false readings concerning an actual hazard.

Thus, there are few analytic tools available that can be used toquantify and characterize, non-intrusively (not slowing or down-gradingthe testing tempo, along with supporting moving object testingmodalities) and in situ at the low concentrations typically required (inthe low parts-per-billion to low parts-per-trillion range). While manytechniques from material sciences are pertinent, each one hasshortcomings that prevent its widespread adoption in a real-timeproduction setting. More importantly, many measurement techniques, whichmight be considered for use, are unable to adequately distinguishchemicals of interest from interferents; such interferents often beingthe result of human-caused situations, or naturally occurring sources.Furthermore, there may be practical confusion of the significance of adetection event due to many chemicals' dual-use applications. Even underwell controlled conditions, background sources may dominate over thetarget materials of interest. Viable solutions are further complicatedwhen the desire is a single detection technology that needs to detect awide-range of chemicals with significantly different molecularstructures, in multiple phases (solid, liquid and/or gas) of matter,instead of just a few closely related species in a single phase ofmatter. Therefore, a way to easily and affordably distinguish differentsources would be of practical value to chemical and materials detection.In contrast to the prior art, the DES technique of the present inventionoffers agility in the range of detectable species, in all phases (solid,liquid and gas) of matter, and has a unique tolerance to interferents.

SUMMARY OF THE INVENTION

The invention relates to a method of detecting the presence of amolecular species in a sample utilizing one or more frequencies ofelectromagnetic radiation, including frequencies matched to themolecular species' rovibrational energy levels, for perturbing therovibrational density of states of the molecular species (hereinafterthe “matched frequencies”). The method, which utilizes means forassessing the spectral response of the molecular species in itsperturbed and unperturbed states and for assessing the presence of themolecular species in the sample, includes: assessing the rovibrationaldensity of states of the molecular species as manifested by its spectralresponse in at least one region of the electromagnetic spectrum;assessing the perturbed state of the molecular species by perturbing therovibrational density of states of the molecular species usingfrequencies of electromagnetic radiation selected from the matchedfrequencies and determining the effects of the perturbation on thespectral response of the rovibrational density of states of themolecular species; and assessing the effect the perturbation had on themolecular species using its perturbed and unperturbed spectralresponses. Assessing the rovibrational density of states of themolecular species (as manifested by its spectral response in the atleast one region of the electromagnetic spectrum) includes interrogatingthe molecular species with electromagnetic radiation in the at least oneregion of the electromagnetic spectrum to determine an unperturbedspectral response of the rovibrational density of states of themolecular species. Assessing the spectral response of the perturbedrovibrational density of states of the molecular species includesilluminating the molecular species with electromagnetic radiationfrequencies selected from the matched frequencies and interrogating themolecular species with electromagnetic radiation in the at least oneregion of the electromagnetic spectrum to determine a perturbed spectralresponse of the rovibrational density of states of the molecularspecies. The means includes means for determining the change between thespectral response of an unperturbed and a perturbed rovibrationaldensity of states of the molecular species and further includingdetermining the change between the spectral response of the unperturbedand the perturbed rovibrational density of states of the molecularspecies. Though stated in a particular order, no representation is madeor intended that this order is always necessary.

The method further utilizes means for determining the concentration ofthe molecular species in the sample, and determining the concentrationof the molecular species in the sample (which may contain one or moremolecular species). More specifically, this uses: the relativedifference between the spectral response of the unperturbed and theperturbed rovibrational density of states of a molecular species in asample used to determine the concentration of the molecular species inthe sample; the relative response of the molecular species within thesample at a known power of frequencies of electromagnetic radiationselected from the matched frequencies for perturbing the rovibrationaldensity of states of the molecular species in the sample; and knownconditions for assessing the spectral response of the molecular speciesin its perturbed and unperturbed states and relating the molecularspecies' response to a library of calibrated responses collected underthe same conditions from known concentrations of the molecular species.The method includes: assessing the rovibrational density of states ofthe molecular species as manifested by its spectral response in at leastone region of the electromagnetic spectrum under known assessmentconditions; assessing the perturbed state of the molecular species byperturbing the rovibrational density of states of the molecular speciesusing by using known powers of frequencies of electromagnetic radiationselected from the matched frequencies and determining the effects of theknown perturbation on the spectral response of the rovibrational densityof states of the molecular species; and assessing the effect theperturbation had on the molecular species using its perturbed andunperturbed spectral responses as related to a pre-compiled library ofcalibrated responses from known concentrations of the molecular species.

The method also includes detecting the presence of at least oneadditional molecular species in a sample (“additional molecularspecies”) utilizing one or more frequencies of electromagneticradiation, including frequencies matched to the additional molecularspecies' rovibrational energy levels, for perturbing the rovibrationaldensity of states of the additional molecular species (“additionalmatched frequencies”). This includes: assessing the rovibrationaldensity of states of the additional molecular species as manifested byits spectral response in at least one region of the electromagneticspectrum; assessing the perturbed state of the additional molecularspecies by perturbing the rovibrational density of states of theadditional molecular species using frequencies of electromagneticradiation selected from the additional matched frequencies anddetermining the effects of the perturbation on the spectral response ofthe rovibrational density of states of the additional molecular species;and assessing the effect the perturbation had on the additionalmolecular species using its perturbed and unperturbed spectralresponses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the MW-IR double resonance interrogation of a targetmolecule with microwave energy.

FIGS. 2A and B are rovibrational transition diagrams between twovibrational singlet states for conventional infrared absorption (FIG.2A) and for the microwave-infrared double resonance technique of thepresent invention (FIG. 2 B).

FIGS. 3A and B illustrate the “DMIRS” (defined below) effect. FIG. 3Ashows the effect on the raw IR spectrum of DMMP; while FIG. 3B shows the“processed” spectrum (the quotient of the RF on/RF off spectra). This isa preferred means for visualizing the effect.

FIG. 4 illustrates a schematic of the apparatus of the present inventionin association with the DMIRS testing of solids and liquids.

FIG. 5 illustrates a schematic of the apparatus of the present inventionin association with the DMIRS testing of gas phase samples.

FIGS. 6A and B show two versions of the liquid cell that would be usedwith, respectively, the apparatus of FIG. 4 (FIG. 6A) and the apparatusof FIG. 5 (FIG. 6B) instead of the gas cell.

FIG. 7 shows the effect of pulse width on DMMP(dimethyl-methylphosphonate) for microwave excitation at 9.698 GHz.

FIG. 8 shows the effect of pulse width on thiodiglycol with 13.000 GHzmicrowave pump photons.

FIG. 9 shows the effect of pulse width on thiodiglycol with 13.250 GHzmicrowave pump photons.

FIG. 10 shows the response of the explosive RDX to 3 GHz pump radiationwith 32 (blue graph) and 192 (red graph) ns pulses. The heightenedresponse at 32 ns is due to matching the resonance condition of theexcited state (specific bond, specific vibrational states).

FIG. 11 shows the DMIRS results for RDX to 3 GHz pump radiation, outsidein full sun, using 32 ns pulses and the microwave frequencies indicatedin the legend.

FIG. 12 shows the pure component spectra of thiodiglycol (red) and DMMP(blue) obtained from the National Institute of Standards (NIST).

FIG. 13 (upper panel) shows the microwave on and off spectra forthiodiglycol, and (lower panel) DMIRS quotient for thiodiglycol at 13.0GHz, 32 ns pulse width. The blue bars indicate transition suppressions,and the red bar indicates an enhancement.

FIGS. 14A-F show thiodiglycol modulation as function of pulse width. 32ns pulses (blue) and 96 ns pulses (red) are compared at microwavefrequencies 13.0 GHz to 15.5 GHz.

FIG. 15 is a table showing an experimental matrix and results forDMMP/Thiodiglycol mixture determination wherein (+)=peak enhancement,(−)=peak suppression, and (N.O.) represents no observable change.

FIGS. 16A and B demonstrate the separability of the thiodiglycol andDMMP responses by using pulse widths and microwave frequencies optimizedfor particular transitions. The top panel compares pure thiodiglycol toDMMP and a 50/50 thiodiglycol/DMMP mixture with 32 ns laser pulses at13.0 GHz microwave excitation frequency which parameters excite thethiodiglycol response while generating virtually no response from DMMP.The bottom panel is the same comparison for a 256 ns pulse width at4.116 GHz which parameters excite the DMMP response while generatingvirtually no response from thiodiglycol. In both cases the mixtureresponse closely matches the response of the pure component being testedfor, while the other species shows minimal response.

FIG. 17 shows the DMIRS response of urea nitrate (UN) at 210 nm. Thelegend is the RF excitation frequency in GHz.

FIGS. 18A and B show the DMIRS response (raw data in A, quotient in B)for 5W-30 motor oil using the UN excitation parameters.

FIGS. 19A and B show the DMIRS response (raw data in A, quotient in B)for automotive door foam using the UN excitation parameters.

FIG. 20 shows the DMIRS response (quotient) at 1550 cm⁻¹ for UN isclearly visible on a laboratory (gold) substrate, on a rubber substrate,and on a rubber substrate with a 5W-30 motor oil coating.

OVERVIEW OF THE TECHNIQUE

Polyatomic molecules consist of atoms joined together by variousstrength bonds based on the electronic configuration of their respectiveelectron clouds. The atoms or molecular sub-groups in a material arefree to vibrate or oscillate with respect to one another, as would beexpected of a bound mechanical system. With reference to FIG. 1, whereinthe spheres represent different types of atoms and C3 and S4 refer topossible rotation and vibration axes. (The MW-IR interrogation,discussed below, is also schematically illustrated.) At the simplestlevel, the molecule can be thought of as a collection of atoms boundtogether with springs. The molecules also have rotational degrees offreedom (these rotational modes are strongly influenced by the state ofmatter, being least constrained in gas form and most constrained insolid form). These rotational and vibrational modes are collectivelyreferred to as “rovibrational” modes. The molecular bonds in polyatomicmolecules have distinct spectral signatures based on the energy of thebond (analogous to spring stiffness in a conceptual model) as well asthe rovibrational mode (type of motion and frequency).

Under the Differential Excitation Spectroscopy (herein after “DES”)umbrella there are a number of technique variants based on methods ofprobing the rovibrational states. Two such variants will be referred toherein as Differential Microwave Excitation IR Spectroscopy (hereinafter“DMIRS”) and Differential Excitation Raman Spectroscopy (hereinafter“DERS”). The skilled practitioner will recognize that these two specificvariants represent a subset of the possible applications of the DEStechnique. DMIRS is a very practical application in that it uses RFenergy to excite the rotational modes (typically from about 100 MHzthrough 20 THz, depending upon the state of matter, size, shape andsymmetry of the molecule) and IR spectroscopy to probe the vibrationalresponse. In a preferred embodiment, the RF energy is in the microwaveregion (100 MHz to 300 GHz) in order to take advantage of atmospherictransmission windows for applications with significant standoffrequirements. DMIRS is only applicable to molecules that are IR-active(i.e. possess at least an instantaneous dipole moment) since IR spectraldata is required; some molecules or modes are not IR active. For suchmolecules, a common measurement approach is Raman spectroscopy, which isbased on photon scattering from the molecule. Hence, the extension ofDES to IR-inactive materials is to excite the rotational modes asdescribed previously and probe the vibrational modes with the Ramantechnique—the above named DERS technique. For the purpose ofillustrating the physics and applications of the DES technique, specificexamples using the DMIRS technique are discussed below as illustrative,but not limiting, examples. Thus, the invention is not limited to theuse of either IR or microwave radiation.

Background Physics of DMIRS

The DMIRS effect is a novel technique that enables molecularrovibrational states to be directly probed using relatively simplepieces of equipment. It is a novel adaptation of pump-probespectroscopic techniques being applied to the molecular rovibrationalstates. Because of the low energies of the rotational modes, microwavephotons are used as the pump source between rotational modes (J states)and higher-energy IR photons are used as the probe mechanism, as theymatch the energy differences between the vibrational energy levels (vstates) as depicted in FIGS. 2A and B.

The transition mechanism depicted in FIG. 2A represents conventional IRabsorption involving one vibrational level transition (v″→v′) over amanifold of rotational states (illustrated as J″≦4 and J′≦4) underarbitrary thermal equilibrium of the molecule. FIG. 2B represents theDMIRS effect, in which one or more of the lower-state rotational levelsis purposely excited (perturbed) at a MW frequency in resonance with itsquantum-mechanically allowed rotational transition (i.e., pump), while asecond photon source (i.e., probe) causes a resonance transitioninvolving one vibrational level; hence, a double resonance effect. Witha significant population of rotationally-perturbed states affected bythe resonance conditions for MW excitation (FIG. 2B), the net effect onobserving (or probing) the infrared absorption (or reflectance) spectrumis a change in the shape and intensity of spectral bands correspondingto the IR resonance of the vibrational transition due to an enhancementor attenuation of rovibrational transition probabilities andstate-to-state lifetimes as compared with pure IR spectroscopy (FIG.2A). This is most clearly illustrated when comparing the J″=2 and J″=3states in FIG. 2A and FIG. 2B, it is clear that the DMIRS effect hasreduced the population of the J″=2 to J′=2 state and increased thepopulation of the J″=3 to J′=3 state, i.e. spectral suppressions andenhancements are observed in the raw data.

Because the technique compares the changes between the unexcited andexcited IR spectra, the vagaries of the underlying IR spectrum (which isa convolution of the incident IR energy and spectral responses of thesubstrate and surface contaminants) are unimportant, as the process ofcomparing the differences results in a self-referencing technique. Thisavoids a common problem with IR spectroscopy. An example of the effectis shown in FIGS. 3A and B, where the gross spectral modifications dueto the DMIRS effect are seen in FIG. 3A (this is the physicalmanifestation of the effect), and the DMIRS quotient (“RF on” signalnormalized by the “RF off” signal) is shown in FIG. 3B. The DMIRSquotient is an easy mechanism for visualizing the enhancement andsuppressions caused by the effect as well as the locations of the effectwithout the complications of underlying raw spectral features. This ishow the self-referencing is accomplished.

In conventional IR spectroscopy, the result is an absorption ortransmission graph—a 1D representation of the convolution of the densityof states at a specific temperature (the distribution is a function oftemperature) and the transition probabilities for each of the possiblestates. It is clear from the discussion of the previous paragraph thatDES uses the probe wavelength (λ) and adds another dimension to thecharacterization of the excited states: the frequency of the pump orexcitation photons (ν), which perturb the rovibrational density ofstates. Our research has demonstrated yet another strong dimension tothe process: the duration of the probe pulse (τ). These multipledimensions for characterization of the material provide built-inrobustness to the technique, thus improving the specificity andreliability of the results and are important detection parameters.

The detection parameters may be determined in two ways. First, abrute-force search of the (λ, ν, τ) parameter space may be conducted.This can be very difficult because the optimal conditions may show highfinesse, i.e. the allowable error around the optimal value may be verysmall, necessitating a very fine grid and hence a great deal of time forthe search. Second, the molecule of interest is modeled, typically as anab initio calculation to determine the shape of the potential energysurface and hence the energy levels and required wavelength andfrequency parameters. This is the preferred approach because it directsthe work efficiently. However, this work is not trivial and thestructure of some molecules may exceed the capabilities of availablemodeling tools.

Early work performed with the above technique used CW illumination andFTIR spectrometers as the detection technique. Recent availability ofQuantum Cascade Lasers has made it possible to examine a materials theresponse to pulsed probe illumination. The variable pulse width of theQCL allows the lifetimes to be evaluated, as the DMIRS effect issignificantly enhanced when the IR probe beam pulse duration iscomparable to (in resonance with) the state lifetime of the vibrationalmode being probed.

Description of the Preferred Embodiments and Test Data

With this invention, we have developed a novel molecular conditioningtechnique which allows the density of states of a molecule to beperturbed from a normal ground state distribution through theapplication of a pump radiation field. The pump radiation field, subjectto the normal constraints of transition probability and absorptioncross-section, preferentially alters the molecular rotational andvibrational states (again, the rovibrational states) in favor ofhigher-order modes. This perturbation of the density of states isphysically manifested by alterations to the spectrum for the material,with certain portions of the spectrum being strengthened (enhanced) orweakened (suppressed), depending on the applied perturbation. Thesechanges in the spectrum are a sensitive indicator of the underlyingmolecular species rovibrational states, as a correctly appliedperturbation will force the molecule into another state. Thisdistribution of states is highly specific to a molecular species, andsimilar, but not identical molecular species would not be expected tohave the same distribution of states. Hence this technique is asensitive probe into the detailed density of states for a specificmolecular species and is an orthogonal measurement to conventionalspectroscopy, as the technique probes more parameters than the groundstate distribution. Its implicit reliance on a unique density of statesmakes it dramatically less susceptible to confusion by similar molecularspecies (e.g., interferents). It is possible to reach more highlyexcited states by either using higher energy photons or by applyingmultiple lower energy photons to reach these states. For a variety ofpractical reasons, such as the atmospheric attenuation, in DMIRSapplications microwave energy is the preferred form of pump radiation.

A microwave region of interest for a preferred embodiment of thetechnique is between 100 MHz through 300 GHz and encompasses thefrequency band containing the fundamental rotational resonancefrequencies of many molecules composed of carbon, nitrogen, oxygen andsulfur. As an inherently differential technique, this novel approach isintrinsically self-referencing, providing a spectroscopic signature thatshows high immunity to spectral interference from background andradiation source variations. In a preferred implementation, the DMIRSresponse is calculated as the quotient of the “microwave on” and“microwave off” spectra, i.e. the spectra collected with and without thepump (or perturbation) radiation source being active. There are a seriesof probe wavelengths, pump frequencies, and probe pulse durationparameters that provide a multi-dimensional characterization of amolecule's excited state energy structure. The essential value of thishigher-dimensionality signature is that the probability of truedetection is higher and background interference less important.

In practice, as mentioned above, the proper combination of spectralregions (e.g., microwave-IR spectral regions) can be determinedempirically by scanning various combinations of electromagneticradiation (e.g., the optical and microwave radiation) to determine theresponses and the unique signature. Alternatively, as also mentionedabove, computational modeling of the molecule to determine its structureand potential energy surface function can be used to determineappropriate combinations of electromagnetic radiation frequencies.

The basic DMIRS apparatus 11 for use in association with testing solidsand liquids, as is set forth in FIG. 4, includes detector 13, pump 15,and probe 17. As illustrated, detector 13 is connected to control andanalysis computer 19, which includes a screen 21 for displaying theresults of test on a sample including at least one molecular species. Asis also illustrated, pump 15 is coupled to pump signal generator andamplifier 23. One of the benefits of the present novel method is that itmay use off the shelf components. Thus, by way of example (but notlimitation): detector 13 is a MCT detector capable of detecting MWIR andLWIR radiation; pump 15 is an RF emitter (e.g., antenna or horn); probe17 is a quantum cascade laser; control and analysis computer 19 is acommercial personal computer; display/screen 21 provides a mechanism forcontrol inputs and from the user, and display of results (this isgenerally considered part of the computer); and pump signal generatorand amplifier 23 is a commercial RF frequency generator with amplifiersto increase the emitted pump radiation power. Control and analysiscomputer includes one or more data bases, including one in which alibrary of responses to the DES technique is stored.

In a mode of operation the rovibrational density of states of a sample24 (e.g., a molecular species) is assessed by exposing it toelectromagnetic radiation 25 from probe 17 to determine an unperturbedresponse of the molecular species. The response signal 27 is detected bydetector 13 and transmitted to control and analysis computer 19.Perturbing the rovibrational density of states of sample 24 byilluminating it with one or more frequencies of electromagneticradiation 29, including frequencies matched to the molecular speciesrovibrational energy levels, is affected by pump signal generator andamplifier 23 and pump 15. The perturbed state of the molecular speciesis assessed (interrogated) by probe 17 and detector 13 and transmittedto control and analysis computer 19 where the effect the perturbationhad on the molecular species (using its perturbed and unperturbedspectral response) is assessed. Control and analysis computer includes aroutine for determining the change between the spectral response of theunperturbed and the perturbed rovibrational density of states of themolecular species. The results may be displayed on display/screen 21.

The difference between the spectral response of the unperturbed and theperturbed rovibrational density of states of a molecular species in asample may be used, by a routine(s) in control and analysis computer 19to determine the concentration of the molecular species in the sample.The methodology uses the response of the molecular species within asample at a known power of frequencies of electromagnetic radiationselected from the matched frequencies for perturbing the rovibrationaldensity of states of the molecular species in the sample and knownconditions for assessing the spectral response of the molecular speciesin its perturbed and unperturbed states and relating the molecularspecies' response to a pre-compiled library of calibrated responsescollected under the same conditions from known concentrations of themolecular species. The library, not shown, is stored in control andanalysis computer 19. The method includes: assessing the rovibrationaldensity of states of the molecular species as manifested by its spectralresponse in at least one region of the electromagnetic spectrum underknown assessment conditions; assessing the perturbed state of themolecular species by perturbing the rovibrational density of states ofthe molecular species using known powers of frequencies ofelectromagnetic radiation selected from the matched frequencies anddetermining the effects of the known perturbation on the spectralresponse of the rovibrational density of states of the molecularspecies; and assessing the effect the perturbation had on the molecularspecies using its perturbed and unperturbed spectral responses asrelated to the pre-compiled library.

While the foregoing is in reference to a sample of a single molecularspecies, apparatus 11 (as well as apparatus 31, apparatus including cell43 and apparatus including cell 53, all discussed below) and themethodology of the present invention can be used to detect the presenceof one or more additional molecular species included in a sample.

For the testing of gas phase samples the apparatus 31, as schematicallyillustrated in FIG. 5, can be used. In addition to detector 13, pump 15,probe 17, control and analysis computer 19, display/screen 21, and pumpsignal generator and amplifier 23, apparatus 31 includes a gas cell 33,preferably a multi-pass design (e.g., a Pfund, White or Heriott cellgeometry (or a functional equivalent)) to increase the effective pathlength while maintaining compactness. The operation is the same asdescribed above with regard to the apparatus of FIG. 4. Matchedfrequency radiation 29 takes the form of a wide beam to ensure that theentire volume of gas cell 33 is available to be probed.

For the testing of bulk liquid samples, the sample 24 of FIG. 4 or thegas cell 33 of FIG. 5 may be replaced by a specially designed liquidcell, optimized to ensure that the volume of the cell is available to beprobed by not exceeding the penetration depths of the pump or proberadiation. With the cell in FIG. 6A replacing the sample 24 of FIG. 4,cell 43 has a window 45 that is transparent to both pump and proberadiation and has a reflective substrate (e.g., gold) 47, and may be ofeither a single reflection or multiple reflection design. Alternatively,as the transmission windows for pump and probe radiation may beincompatible with available window materials, FIG. 6B (which replacesthe gas cell 33 of FIG. 5) separates the pump window 55 and probe window57 of cell 53, so that the pump and probe radiation will be transmittedthrough different windows. As before, the reflective substrate 47 inconjunction with probe window 57 may be configured in either a single ormultiple reflection design. Multiple reflection designs have theadvantage of the gas cell 33, in that they increase the mass of materialprobed, thus increasing ultimate device sensitivity.

The equipment set forth above is intended to be representative, notlimiting. Also, such equipment could be used in other DES applications.

Test Results

Different molecules and even different bonds and states of a moleculewill, in principle, have different sets of optimal excitationparameters. This is shown in the following sections for four differentmolecules: DMMP (dimethyl-methylphosphonate), thiodiglycol, RDX (anexplosive) and urea nitrate. The TNT test data set forth in provisionalapplication Ser. No. 61/877,144, incorporated by reference, was takenwith a CW source and, therefore had lower modulation (no pulse durationmodulation).

Liquid: DMMP

FIG. 7 shows the DMIRS quotient for DMMP under the same excitationconditions (9.698 GHz microwave frequency). This figure shows the lasertransition points at 1006 and 1220 cm⁻¹ (the discontinuities at thesepoints should be ignored as this is where the probe laser was changed).The figure demonstrates: (1) the very high modulation enhancements thatare possible when the pulse width is correctly selected; and (2)different bonds with the same microwave excitation frequency can havevery different lifetimes. This latter effect is evident when noting thesingle, very narrow and strong enhancement at ˜1175 cm⁻¹ seen at 32 ns(the green plot) versus the wide variety of enhancements seen at 256 ns(the blue plot). Comparing the two plots, one can also see some of thesame features (peak locations) in both sets of data, although there maybe reversal between enhancement and suppression. There is clearly anenormous amount of complexity and information available in the DMIRSresponse; this information provides enormous insight into the energystates and transition lifetimes of a molecule of interest. Because DMMPis a more complex molecule, the full material response is also morecomplex.

Liquid: Thiodiglycol

Thiodiglycol, also a liquid, exhibits the same qualitative pulse widthenhancements as DMMP though the specific details are different. FIG. 8shows the enhancement at 13 GHz for 32 (blue graph) and 96 (red) nspulses, with the latter signal being distinguishable from noise onlywith a priori knowledge of the feature locations (such as theinformation seen in the enhanced 32 nm spectrum). The suppression blipat approximately 1080 cm⁻¹ in the red curve is an example of the weakerresponse. FIG. 9 gives a glimpse into the richness of structure that acomplex molecule can demonstrate: at 13.250 GHz, the structure ofexcited states has changed (e.g. consider the features in the 1075-1090cm⁻¹ range: both pulse widths create enhancements, although they aredifferent and may shift). In the ˜1325 cm⁻¹ range pulse widths drivedifferent shape enhancements (meaning that adjacent features showdifferent resonances) and the suppression seen at 13 GHz (FIG. 8) hasnow become varying enhancements.

Solid: RDX

The pulse width dependence of the DMIRS effect is not confined to liquidsamples; it is a fundamental parameter also affecting solid-state matterfor weakly-bound molecular crystals and gases. While the specificdetection parameters will be affected by the state of matter (e.g. in asolid, the crystal lattice will exert damping forces which affectrotations and, hence, the correct microwave frequencies), the fact thatthe pulsed width needs to be in resonance with the states being probedis unchanged. As a specific example, consider FIG. 10 showing that for aseries of states of RDX corresponding to wavenumbers between ˜1250 and1450 cm⁻¹, for the specific excitation frequency of 3 GHz, thedifference between a signal which barely—if at all—stands out from thenoise and a signal which is unmistakable and has a very highsignal-to-noise ratio is the choice of a 32 nm pulse width rather than a192 ns pulse width. In hindsight, one can see these features at thelonger pulse width, but the low modulation strength makes it verydifficult to use this information in any practical way.

As a reminder that all three parameters (wavelength, frequency and pulsewidth) need to be selected correctly, FIG. 11 shows the change inmodulation and, for that matter, the types of changes (enhancement orsuppression and the location of these effects) as the microwavefrequency changes. If the parameter set is far from optimal, theresponse will be weak. See the blue curves (representing 1 and 15 GHz).When optimized, even in solids, the modulation can be significant, as isevident from the cyan colored curve (3 GHz).

Practical Ramifications: Mixtures

A common difficulty in spectroscopic analysis is resolving individualmolecular species in complex mixtures. Cases where spectral overlap ishigh make resolving individual components from the convolved spectradifficult. Infrared spectra of mixtures are comprised of peaks from eachcomponent making separation of peaks due to individual components achallenge. An example of this type mixture is thiodiglycol (CAS111-48-8) and dimethyl-methylphosphonate (CAS 756-79-6) (DMMP). Standardinfrared spectra for both components are presented in FIG. 12. Here thedegree of overlap precludes elucidation of either species as bothdemonstrate simultaneous absorptions at nearly the same frequencies.

Common techniques for resolving mixtures generally involve postprocessing data and the ability to extract component information fromadditive individual spectra. This involves the approximation that themixture spectrum is the sum of the pure components multiplied by theconcentration of each species. This approximation is appropriate incases where intermolecular interactions are low. However, in liquidsthese interactions, such as hydrogen bonding, cause shifts in spectralfeatures, thus complicating the extrapolation to pure component spectra.This method is further complicated if the mixture is not known, or is inthe presence of a varying background.

An alternate approach is to take advantage of enhanced selectivitygained from multi-dimensional interrogation of the sample via the DMIRStechnique of the present invention. For a given species there are a setof microwave frequencies that can be observed as rovibrational states inthe DMIRS spectrum. Within these states there is a resonant pulse width,which produces an increase in the observed modulation. By optimizing thepulse width for a particular rovibrational state, the transition ofinterest can be enhanced, or suppressed. This phenomena is shown in FIG.13 for a liquid sample of thiodiglycol. The figure demonstrates theenhancement and suppression in the raw on and off spectra (top figure)as well as the resulting quotient (on/off) (bottom figure). The bluebars indicate transition suppressions, while the red bar indicates anenhancement.

The influence of the pulse width on differential MW spectrum isdemonstrated in the comparison of DMIRS quotients collected at two pulsewidths for a variety of pump frequencies ranging from 13-15.5 GHz (FIGS.14A-F). Here thiodiglycol was excited using 32 (blue curves) and 96 (redcurves) ns pulses and microwave frequencies were determined throughprior empirical observation. The 13.0 GHz data in the upper left corner(FIG. 14A) shows almost 20% modulation for the feature spanning 1200cm⁻¹ to 1300 cm⁻¹ at the 32 ns pulse width, but far lower modulation at96 ns. Similar qualitative effects are seen for the other pumpfrequencies, although the specific details are quantitatively differentfor each of the cases. The low modulation at 15.25 and 15.50 GHzsuggests that these excitation frequencies are far from optimal. It isimportant to note that the resonance pulse width may only apply to asingle transition, so optimizing the DMIRS technique for an analyte ofinterest either requires a priori knowledge or a rigorous treatment ofall experimental parameters, but the technique can then be tuned todiscriminate interference, environment, and competing species. As aqualitative measure of determining if a molecular species of interest ispresent, the unique ability to invert a feature from an enhancement to asuppression is useful, and is demonstrated by comparing several featuresin the 13.0 GHz data (FIG. 14A) and at 15.0 GHz FIG. 14D). Resonancecondition serves to enhance specific transitions, as also demonstratedat 13.0 GHz and 15.0 GHz, where the blue curve is almost mirrored aroundthe horizontal axis. FIGS. 8 and 9 were extracted from this data.

Mixture Analysis

Pure thiodiglycol, pure DMMP, and a 50/50 mixture of DMMP/thiodiglycolwere interrogated using three pulse widths and seven microwavefrequencies each optimized for either a DMMP or thiodiglycol transition.The experimental matrix and observed results are shown in the table ofFIG. 15, peak enhancements are indicated with a (+), suppressions with a(−), and if no effect is observed (N.O.).

A comparison of optimal microwave perturbation and pulse width ispresented for the DMMP/thiodiglycol mixture at 13.0 GHz with 32 ns pulsewidth and 4.116 GHz with 256 ns pulses in FIGS. 16A and B. FIG. 16Ashows the previously shown modulation for thiodiglycol at 32 ns pulsewidth. The mixture data closely mirrors the pure thiodiglycol data, withvirtually no response attributable to DMMP. The modulation is lower forthe mixture spectrum (12% as compared to 20%), but this can be ascribedto the mixture being diluted with non-responding DMMP molecules. Theresponse for thiodiglycol is quite broad at the short pulse width, withmost of the spectrum showing some degree of modulation. Similarly, FIG.16B displays the response for DMMP at 4.116 GHz observed with 256 nspulse width. Once again the mixture response closely matches that ofpure DMMP, and has broad response, especially for the low energy portionof the spectrum. In both cases the interesting result may not be theoptimized response of either analyte, but the lack of response from theinterferent.

Interferents

The mixture problem may be viewed as identifying a specific molecule (ormolecules) in a mixture, as was done in the previous example, or asimmunity to interferents, namely: the ability to detect the molecule ofinterest, taking advantage of a priori knowledge about optimalexcitation parameters. The immunity to interferents is demonstratedusing an example of urea nitrate (“UN”). In a laboratory environment,urea nitrate was deposited on a gold substrate (gold being chosen forlaboratory work as the reflectivity of gold is uniform across the IRspectral region). Under these circumstances, optimal detectionparameters can be determined. Based on modeling results, DMIRSmodulation at 1550 cm⁻¹ was predicted and observed experimentally. Thiseffect is shown in FIG. 17, where the DMIRS response at severalcalculated UN excitation or pump frequencies is shown. Using the samedetection parameters, 5W-30 motor oil shows no response, as shown inFIGS. 18A and B. (FIG. 18A shows the raw IR spectral data; FIG. 18Bshows the calculated quotient.) Rubber automotive door foam similarlyshows no response, as shown in FIGS. 19A and B. (FIG. 19A shows the rawIR spectral data; FIG. 19B shows the calculated quotient.) Consideringthe sequence of FIGS. 18A and B and FIGS. 19A and B, it comes as nosurprise that FIG. 20 shows that the DMIRS effect clearly allows amolecule of interest to be seen in the presence of non-optimal,real-world substrates and interferent overcoats. This can be seen byconsidering the 1550 cm⁻¹ response in FIGS. 17 and 20 and the absence ofsaid response in FIGS. 18B and 19B (the substrate and interferents).

Whereas the drawings and accompanying description have shown anddescribed the preferred embodiments of the present invention, it shouldbe apparent to those skilled in the art that various changes may be madein the forms and uses of the invention without affecting the scopethereof.

We claim:
 1. A method of detecting the presence of a molecular speciesin a sample; the method utilizing means for generating one or morefrequencies of electromagnetic radiation, including frequencies matchedto the molecular species' rovibrational energy levels, for perturbingthe rovibrational density of states of the molecular species(hereinafter the “matched frequencies”); the method further utilizingmeans for assessing the spectral response of the molecular species inits perturbed and un-perturbed states and for assessing the presence ofthe molecular species in the sample; the method including: assessing therovibrational density of states of the molecular species as manifestedby its spectral response in at least one region of the electromagneticspectrum; assessing the perturbed state of the molecular species byperturbing the rovibrational density of states of the molecular speciesusing frequencies of electromagnetic radiation selected from the matchedfrequencies and determining the effects of the perturbation on thespectral response of the rovibrational density of states of themolecular species; and assessing the effect the perturbation had on themolecular species using its perturbed and un-perturbed spectralresponses.
 2. The method as set forth in claim 1, wherein assessing therovibrational density of states of the molecular species as manifestedby its spectral response in the at least one region of theelectromagnetic spectrum includes interrogating the molecular specieswith electromagnetic radiation in the at least one region of theelectromagnetic spectrum to determine an un-perturbed spectral responseof the rovibrational density of states of the molecular species.
 3. Themethod as set forth in claim 1, wherein assessing the spectral responseof the perturbed rovibrational density of states of the molecularspecies includes illuminating the molecular species with electromagneticradiation frequencies selected from the matched frequencies andinterrogating the molecular species with electromagnetic radiation inthe at least one region of the electromagnetic spectrum to determine aperturbed spectral response of the rovibrational density of states ofthe molecular species.
 4. The method as set forth in claim 1, whereinthe means includes means for determining the change between the spectralresponse of an un-perturbed and a perturbed rovibrational density ofstates of the molecular species and further including determining thechange between the spectral response of the un-perturbed and theperturbed rovibrational density of states of the molecular species. 5.The method as set forth in claim 4, further including means fordetermining the concentration of the molecular species in the sample,and further including determining the concentration of the molecularspecies in the sample.
 6. The method as set forth in claim 1 wherein themeans includes means for determining the change between the spectralresponse of an un-perturbed and a perturbed rovibrational density ofstates of the molecular species, and wherein the method furtherincludes: assessing the rovibrational density of states of the molecularspecies as manifested by its spectral response in the at least oneregion of the electromagnetic spectrum by interrogating the molecularspecies with electromagnetic radiation in the at least one region of theelectromagnetic spectrum to determine an un-perturbed spectral responseof the rovibrational density of states of the molecular species;assessing the perturbed rovibrational density of states of the molecularspecies by illuminating the molecular species with electromagneticradiation frequencies selected from the matched frequencies andinterrogating the molecular species with electromagnetic radiation inthe at least one region of the electromagnetic spectrum to determine aperturbed spectral response of the rovibrational density of states ofthe molecular species; and determining the change between the spectralresponse of an un-perturbed and a perturbed rovibrational density ofstates of the molecular species.
 7. The method as set forth in claim 1,where the difference between the spectral response of the un-perturbedand the perturbed rovibrational density of states of a molecular speciesin a sample is used to determine the concentration of the molecularspecies in the sample; the method using the response of the molecularspecies within the sample at a known power of frequencies ofelectromagnetic radiation selected from the matched frequencies forperturbing the rovibrational density of states of the molecular speciesin the sample and known conditions for assessing the spectral responseof the molecular species in its perturbed and un-perturbed states andrelating the molecular species' response to a library of calibratedresponses collected under the same conditions from known concentrationsof the molecular species; the method including: assessing therovibrational density of states of the molecular species as manifestedby its spectral response in at least one region of the electromagneticspectrum under known assessment conditions; assessing the perturbedstate of the molecular species by perturbing the rovibrational densityof states of the molecular species using by using known powers offrequencies of electromagnetic radiation selected from the matchedfrequencies and determining the effects of the known perturbation on thespectral response of the rovibrational density of states of themolecular species; and assessing the effect the perturbation had on themolecular species using its perturbed and un-perturbed spectralresponses as related to a pre-compiled library of calibrated responsesfrom known concentrations of the molecular species.
 8. The method as setforth in claim 1, where the matched frequencies are in the range ofradio frequencies and the spectral responses are determined in the rangeof IR frequencies.
 9. The method as set forth in claim 1, furtherincluding detecting the presence of at least one additional molecularspecies in a sample (hereinafter “additional molecular species”); themethod utilizing one or more frequencies of electromagnetic radiation,including frequencies matched to the additional molecular species'rovibrational energy levels, for perturbing the rovibrational density ofstates of the additional molecular species (hereinafter the “additionalmatched frequencies”); the method includes: assessing the rovibrationaldensity of states of the additional molecular species as manifested byits spectral response in at least one region of the electromagneticspectrum; assessing the perturbed state of the additional molecularspecies by perturbing the rovibrational density of states of theadditional molecular species using frequencies of electromagneticradiation selected from the additional matched frequencies anddetermining the effects of the perturbation on the spectral response ofthe rovibrational density of states of the additional molecular species;and assessing the effect the perturbation had on the additionalmolecular species using its perturbed and un-perturbed spectralresponses.